Plasma renin activity (PRA), aldosterone (Aldo), 17alpha-hydroxyprogesterone (17-OHP) and testosterone (T), together with urine sodium, pregnanetriol, 17-oxosteroids and the 11-oxygenation index (11-OH) were estimated in 23 patients (age 5.7--18 yrs.) with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency during glucocorticoid treatment. Elevated PRA levels (1400--17200 ng Al/l/hr) were found in 13 out of 15 patients with a history of salt loss. Three non-salt losers showed high PRA levels and in the remaining 5 the levels were in the upper normal range (540--900 ng Al/l/hr). Plasma Aldo levels were normal (25--620 pmol/l) in 18 patients and slightly elevated (690--2360 pmol/l) in 5. While these results indicate persistent impairment of sodium homeostasis in CAH patients, no significant correlations between log. PRA, log. Aldo and urinary sodium excretion were found. Mid-day 17-OHP levels ranged from 9 to 117 nmol/l and T from 0.3 to 18.0 nmol/l. Neither the 17-OHP nor the T results correlated well with the clinical assessment of therapeutic control. The results of the urinary steroid determinations showed better agreement with the clinical assessment of treatment and the 17-oxosteroid, pregnanetriol and 11-OH index results appeared to be better discriminants between good and poor control. Twelve of the patients with a history of early salt loss were reinvestigated after one month's treatment with oral 9 alpha-flurohydrocortisone (0.05 mg/day). PRA was reduced in 7 patients and 17-OHP fell in 10 patients. No consistent changes were found in Aldo, T, or urinary sodium and steroid excretion during this low-dose mineralocorticoid treatment.